Spray applications are known which involve the use of an accelerating agent introduced into a composition after conveyance through a hose. In particular, the composition being pumped through the hose is a hydratable cementitious particle slurry or suspension which is expelled through a spray nozzle onto a substrate. For example, such accelerating agents are used in the spray-application of concrete (e.g., “shotcrete”) and fire protection coatings for metal substrates such as steel beams and panels.
A particularly successful type of product in the fire protection field is a gypsum-based slurry formulation that contains, in addition to the gypsum binder, a lightweight inorganic aggregate, such as shredded expanded polystyrene, vermiculite, and/or cellulose, along with an air entrainer and other agents. Fireproofing mixes such as the MONOKOTE® brand fireproofing from Grace Construction Products, Cambridge, Mass., are sold as a dry powder. The powder is mixed with water at the job site using a mixer that is situated at ground level. The fireproofing slurry is then pumped through a long hose which sometimes extends to a location 20 to 30 stories above ground level. It is sprayed through a nozzle onto a steel beam, panel, or other construction surface.
As explained by Hilton et al. in U.S. Pat. No. 4,904,503, the slurries must possess a number of important properties for spray application. They must adhere to the metal substrate in both the wet slurry state and after setting. They must set without undue expansion or shrinkage in order to avoid crack formations that could undermine the insulative properties of the coating. The slurry compositions must be able to hold a relatively large quantity of water so that they can be pumped easily and to great heights. Yet, the slurries must retain a consistency so as to avoid segregation or settling of ingredients.
From an economic viewpoint, it is important that the sprayable slurry provide an adequate “yield.” The term “yield” means the volume of applied fireproofing composition, after setting, per given weight of dry mix used to prepare the fireproofing composition slurry.
To increase yield, Hilton et al. disclosed in U.S. Pat. No. 4,904,503 that an acidic set accelerator could be injected into the cementitious slurry to react with a basic material contained therein. The reaction provides carbon dioxide gas for foaming and expanding the slurry composition, thereby providing a substantial increase in yield of the slurry after setting. In the '503 patent, it was suggested that the accelerator be introduced into the slurry close to the nozzle to minimize the potential for hardening of the accelerated mix in the feed line during pumping.
The use of an accelerator has been known to convert the setting time of the slurry mix from about 4 to 12 hours to less than about 15 minutes, and it is often preferred to employ an amount of accelerator sufficient to obtain a setting time of about 5 to 10 minutes and also to obtain a yield increase in the range of 0.1% to 20%.
Volumetric flow metering techniques have been used in the spray application of gypsum-based fireproofing as well as sprayed concrete (shotcrete) to control the amount of set accelerator introduced into the slurry. For example, it is believed that flow meters have been used for introducing an alum accelerator into the hose during the spray-application of gypsum-based fireproofing. As another example, a volumetric flow metering systems are commercially available from Normet Corporation under the trade name Nordoser 900™ and from MBT Pty Ltd of Australia under the trade name Viper Roboshot MkIII for use in sprayed concrete applications.
The present inventors, however, believe that the use of flow meters do not necessarily guarantee accurate monitoring where lengthy hoses are required for application of the slurry onto substrates. A sufficient level of accuracy is especially harder to attain where the distances between the mixer and nozzle are great. For example, a high-rise construction site requires several hundred feet of slurry hose to be employed. This means that several hundred feet of small diameter tubing will also be required for pumping the set accelerator to or near the spray nozzle. Accordingly, the hose path from the mixer at ground level to the spray nozzle may span the distance of 10 to 30 stories or more above ground level. The path usually involves winding and circuitous routes around several obstacles and sharp edges. Typical obstacles for the accelerator tubing include door and wall openings, stairwells, and metal studs. The tubing is susceptible to pinching, kinking, collapsing, leaking, and other conditions that prevent a controlled introduction of set accelerating agent into the slurry.
An objective of the present invention, therefore, is to provide a method and device for accelerator injection and monitoring that provide accurate monitoring of accelerator levels actually introduced into the slurry.
Another objective is to provide an accelerator monitoring method and device that can withstand the corrosive effect of cementitious compositions on metering equipment and pluggage effect on the accelerator injector; that employ cleaning methods using current line cleaning methods; and that involve un-obtrusiveness to the spray operation.
A still further objective is to provide a device that can withstand the hazards of the industrial work environment. For example, the accelerator monitoring device should be able to withstand repeated drops onto hard surfaces without any loss in accuracy or effectiveness.
Finally, an important objective of the present invention is to provide a method and device for obtaining consistently high yield spray application of a settable composition using a set accelerator agent that is injected into the hose-conveyed composition being spray applied.